Column density fractionation ratios plotted against the emission line ratio T_mb( [CII] )/T_mb(H¹³CO ⁺ (1 − 0)) for all models from our parameter space (blue: ER, red: ER    ±    10%, dashed: ER  ±    20%). The symbols follow the coding from Fig. 9. The gray areas denote the regime below a sensitivity limit of T_mb < 0.01    K   km   s^-1. Left panel: N(CO)/N(¹³CO), middle panel: N(C)/N(¹³C), right panel: .